The rheology of volcanic rock avalanches and dense pyroclastic flows is complex, and it is difficult at present to constrain the physics of their processes. The problem lies in defining the most ...suitable parameters for simulating the behavior of these natural flows. Existing models are often based on the Coulomb rheology, sometimes with a velocity‐dependent stress (e.g., Voellmy), but other laws have also been used. Here I explore the characteristics of flows, and their deposits, obtained on simplified topographies by varying source conditions and rheology. The Coulomb rheology, irrespective of whether there is a velocity‐dependent stress, forms cone‐shaped deposits that do not resemble those of natural long‐runout events. A purely viscous or a purely turbulent flow can achieve realistic velocities and thicknesses but cannot form a deposit on slopes. The plastic rheology, with (e.g., Bingham) or without a velocity‐dependent stress, is more suitable for the simulation of dense pyroclastic flows and long‐runout volcanic avalanches. With this rheology, numerical flows form by pulses, which are often observed during natural flow emplacement. The flows exhibit realistic velocities and deposits of realistic thicknesses. The plastic rheology is also able to generate the frontal lobes and lateral levées which are commonly observed in the field. With the plastic rheology, levée formation occurs at the flow front due to a divergence of the driving stresses at the edges. Once formed, the levées then channel the remaining flow mass. The results should help future modelers of volcanic flows with their choice of which mechanical law corresponds best to the event they are studying.
Key Points
Rheology of pyroclastic flows and long runout avalanches
Morphological study
Formation of frontal lobes and levees
Pyroclastic currents are very destructive and their complex behavior makes the related hazards difficult to predict. A new numerical model has been developed to simulate the emplacement of both the ...concentrated and the dilute parts of pyroclastic currents using two coupled depth‐averaged approaches. Interaction laws allow the concentrated current (pyroclastic flow) to generate a dilute current (pyroclastic surge) and, inversely, the dilute current to form a concentrated current or a deposit. The density of the concentrated current is assumed to be constant during emplacement, whereas the density of the dilute current changes depending on the particle supply from the concentrated current and the mass lost through sedimentation. The model is explored theoretically using simplified geometries as proxies for natural source conditions and topographies. It reproduces the relationships observed in the field between the surge genesis and the topography: the increase in surge production in constricted valleys, the decoupling between the concentrated and the dilute currents, and the formation of surge‐derived concentrated flows. The strong nonlinear link between the surge genesis and the velocity of the concentrated flow beneath it could explain the sudden occurrence of powerful and destructive surges and the difficulty of predicting this occurrence. A companion paper compares the results of the model with the field data for the eruption of Merapi in 2010 and demonstrates that the approach is able to reproduce the natural emplacement of the concentrated and the dilute pyroclastic currents studied with good accuracy.
Plain Language Summary
Pyroclastic currents are composed of hot gas and rock fragments. They are very dangerous and their complex behavior makes the related hazards difficult to predict. They are generally formed of two distinct parts: (1) a basal flow that carries ashes and large blocks (up to cubic meters), which is very destructive but follows existing valleys; (2) a dilute part, called pyroclastic surge, that carries ashes in hot turbulent gases. This part is less destructive for infrastructures but it is less confined by the topography, escapes easily from the valleys, and is very dangerous for the inhabitants. A new numerical model has been developed to simulate their emplacement. It reproduces the relationships observed in the field between the surge genesis and the topography. The strong nonlinear link between the surge genesis and the velocity of the flow beneath it could explain the sudden occurrence of powerful and destructive surges and the difficulty of predicting this occurrence. This new model gives promising perspectives for the understanding of pyroclastic current emplacements and for future estimation of related hazards and impacts on the population, the infrastructure, and the environment.
Key Points
A new depth‐averaged model of pyroclastic currents coupling the concentrated and the dilute parts
The model is tested theoretically and reproduces the main characteristics observed in the field
This is a new version of the code VolcFlow developed for the modeling of volcanic flows
Landslide-generated tsunamis at Réunion Island Kelfoun, Karim; Giachetti, Thomas; Labazuy, Philippe
Journal of Geophysical Research: Earth Surface,
December 2010, Letnik:
115, Številka:
F4
Journal Article
Recenzirano
Odprti dostop
Landslides that occur on oceanic volcanoes can reach the sea and trigger catastrophic tsunamis. Réunion Island has been the location of numerous huge landslides involving tens to hundreds of cubic ...kilometers of material. We use a new two‐fluid (seawater and landslide) numerical model to estimate the wave amplitudes and the propagation of tsunamis associated with landslide events on Réunion Island. A 10 km3 landslide from the eastern flank of Piton de la Fournaise volcano would lift the water surface by about 150 m where it entered the sea. The wave thus generated would reach Saint‐Denis, the capital of Réunion Island (population of about 150,000 people), in only 12 min, with an amplitude of more than 10 m, and would reach Mauritius Island in 18 min. Although Mauritius is located about 175 km from the impact, waves reaching its coast would be greater than those for Réunion Island. This is due to the initial shape of the wave, and its propagation normal to the coast at Mauritius but generally coast‐parallel at Réunion Island. A submarine landslide of the coastal shelf of 2 km3, would trigger a ∼40 m high wave that would severely affect the proximal coast in the western part of Réunion Island. For a landslide of the shelf of only 0.5 km3, waves of about 2 m in amplitude would affect the proximal coast.
Lava dome collapses are a major threat to the population living near such volcanoes. However, it is not possible to forecast collapses reliably because the mechanisms are not clearly understood, due ...partly to the lack of continuous observations of such events. To address this need for field data, we have developed new monitoring stations, which are adapted to the volcanic environment. The stations tracked the complete evolution of the 2018–2019 lava dome of Merapi volcano (Indonesia) and the associated pyroclastic density currents. During the 14 months of activity, the stations acquired thermal, high-resolution visual images and movies in stereoscopic configurations. The dome developed on a plateau flanked by steep sides (~ 40°–50°) inside the crater, which was open to the SE. We observed that the dome behaved in a viscous manner (with a viscosity of 10
9
Pa s for the interior to 10
13
Pa s for external parts of the dome) on gentle slopes, and in a brittle way (friction angle ~ 35°, cohesion < 100 kPa) on slopes steeper than 35°. Thus, the lava dome was unable to grow on the outer slopes of the plateau and a significant volume of lava (350–750 × 10
3
m
3
) accumulated and collapsed daily to the SE in relatively small volumes (< 10,000 m
3
), preventing the lava dome from reaching the critical volume necessary for pyroclastic density currents to form and threaten the surrounding population. The cause of the small and frequent collapses was purely gravitational during the dome activity. This suggests that relatively small differences in the summit morphology can control dome evolution, favouring either a lava dome restricted to a small volume and leading to only a minor crisis, or more voluminous dome growth and a catastrophic collapse.
Abstract
Volcanic eruptions can trigger tsunamis, which may cause significant damage to coastal communities and infrastructure. Tsunami generation during volcanic eruptions is complex and often due ...to a combination of processes. The 1650 eruption of the Kolumbo submarine volcano triggered a tsunami causing major destruction on surrounding islands in the Aegean Sea. However, the source mechanisms behind the tsunami have been disputed due to difficulties in sampling and imaging submarine volcanoes. Here we show, based on three-dimensional seismic data, that ~1.2 km³ of Kolumbo’s northwestern flank moved 500–1000 m downslope along a basal detachment surface. This movement is consistent with depressurization of the magma feeding system, causing a catastrophic explosion. Numerical tsunami simulations indicate that only the combination of flank movement followed by an explosive eruption can explain historical eyewitness accounts. This cascading sequence of natural hazards suggests that assessing submarine flank movements is critical for early warning of volcanogenic tsunamis.
The 1888 Ritter Island volcanic sector collapse triggered a regionally damaging tsunami. Historic eyewitness accounts allow the reconstruction of the arrival time, phase and height of the tsunami ...wave at multiple locations around the coast of New Guinea and New Britain. 3D seismic interpretations and sedimentological analyses indicate that the catastrophic collapse of Ritter Island was preceded by a phase of deep-seated gradual spreading within the volcanic edifice and accompanied by a submarine explosive eruption, as the volcanic conduit was cut beneath sea level. However, the potential impact of the deep-seated deformation and the explosive eruption on tsunami genesis is unclear. For the first time, it is possible to parameterise the different components of the Ritter Island collapse with 3D seismic data, and thereby test their relative contributions to the tsunami. The modelled tsunami arrival times and heights are in good agreement with the historic eyewitness accounts. Our simulations reveal that the tsunami was primarily controlled by the displacement of the water column by the collapsing cone at the subaerial-submarine boundary and that the submerged fraction of the slide mass and its mobility had only a minor effect on tsunami genesis. This indicates that the total slide volume, when incorporating the deep-seated deforming mass, is not directly scalable for the resulting tsunami height. Furthermore, the simulations show that the tsunamigenic impact of the explosive eruption energy during the Ritter Island collapse was only minor. However, this relationship may be different for other volcanogenic tsunami events with smaller slide volumes or larger magnitude eruptions, and should not be neglected in tsunami simulations and hazard assessment.
Giant mass failures of oceanic shield volcanoes that generate tsunamis potentially represent a high-magnitude but low-frequency hazard, and it is actually difficult to infer the mechanisms and ...dynamics controlling them. Here we document tsunami deposits at high elevation (up to 132 m) on the north-western slopes of Tenerife, Canary Islands, as a new evidence of megatsunami generated by volcano flank failure. Analyses of the tsunami deposits demonstrate that two main tsunamis impacted the coasts of Tenerife 170 kyr ago. The first tsunami was generated during the submarine stage of a retrogressive failure of the northern flank of the island, whereas the second one followed the debris avalanche of the subaerial edifice and incorporated pumices from an on-going ignimbrite-forming eruption. Coupling between a massive retrogressive flank failure and a large explosive eruption represents a new type of volcano-tectonic event on oceanic shield volcanoes and a new hazard scenario.
The Mount Pelée May 8th, 1902 eruption is responsible for the deaths of more than 29,000 people, as well as the nearly-complete destruction of the city of Saint Pierre by a single pyroclastic ...current, and is, sadly, the deadliest eruption of the 20th century. Despite intensive field studies on the associated deposit, two conflicting interpretations of the pyroclastic current dynamics (either a blast or a simple ash-cloud surge) emergedin the 90’s and have been paralyzing research ever since, leaving numerous unknowns (i.e., source conditions, volume). This study is the first to investigate numerically the May 8th, 1902 pyroclastic current, using the new two-phase version of VolcFlow that simulates more accurately both parts of pyroclastic currents (i.e., the block-and-ash flow and the ash-cloud surge). Physical flow parameters are either extracted from fielddata or estimated empirically when no value was found in the literature. Among the two interpretations, only the simple ash-cloud surge is tested, generated from a block-andash flow initially supplied from the artificially recreated 1902 crater. The block-and-ash flow overflows from the southern V-shaped crater outlet and stays confined into the Rivière Blanche, whereas the ash-cloud surge expands radially and spreads westward,seaward, and eastward, ultimately reaching St Pierre 8 km away, within 330 s. The extent of both parts of the simulated current, as well as the thickness and the direction of the ash-cloud surge are accurately reproduced for a total volume of 32 106 m3, for which a significant part (one third) is deposited in the sea (not recorded in previous studies). Simulations demonstrate that the pear-like shape of the ash-cloud surgedeposit is explained by a late surge production along the Rivière Blanche but also that a blast-like event may be required at the initial stage of the explosion, which in some way reconciles the two conflicting past interpretations. Results also highlight the role played by the topography in controlling transport and deposition mechanisms of such pyroclastic currents especially the lateral spreading of the ash-cloud surge. Our studyimproves the assessment of pyroclastic current-related hazards at Mount Pelée, which could be helpful for future eruptions.
Reconstructing bomb trajectories resulting from Strombolian activity can provide insights into near-surface dynamics of the conduit system. Typically, the high number of bombs involved represents a ...challenge for both automatic and manual bomb identification and tracking methods. Here, we present a method for the automated recognition of hundreds of bombs (100 to 400 depending on the explosion observed) and for the reconstruction of their trajectories in time and 3D space by stereophotogrammetry. The data involve video collected at 30 Hz with two synchronised cameras (Basler 1300-30), separated by 11°, targeting explosions at Stromboli (Aeolian Islands, Italy) in September–October 2012. In total, six data sets were collected for emissions lasting less than 15 s. The 3D reconstructions provided more accurate velocity estimations (error < 10%) than 2D analyses (errors up to 90–100% for bombs moving parallel to the line of sight of the camera). By coupling the measured trajectories with a numerical ballistic model, we show that the method can be used to estimate the directional distribution of bombs and their velocities at the vent (which in this case was 30–130 m s
−1
), the wind velocity (~ 3.5 m s
−1
from the NW) and the drag coefficients (10
−3.5
− 10
−0.5
) of the bombs. The 3D reconstructions also provide a quantification of the directions of explosions and show that explosions can be radial, oriented in a predominant direction of ejection or in several directions; these dispersion patterns can change during a few seconds in a single explosion. We relate the changing directions of ejections to rheological variations in the upper part of the magmatic system probably filled with a mixture of partially crystallised magma which can direct rising slugs along preferential paths.
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